Magnetic disks are commonly used to store data for computer applications. Disks are divided into a number of concentric circular tracks and data is stored along these tracks as individual magnetized portions of the track. A transducer having a flux path and a gap is used to magnetize the track. The gap is passed near the disk. By changing the magnetic flux passing through the gap individual portions of the track are magnetized. The same transducer is also used to read the data off the disk.
It is desirable to maximize the amount of data that can be stored on a disk in order to save space and reduce the number of disks needed to store a particular amount of data. Several methods are currently used to store data on a disk. One method writes or inputs the data onto the disk at a fixed frequency as the magnetic disk rotates at a fixed angular velocity. A major drawback associated with this method relates to data density, which is the amount of data which can be stored per inch of track. Since the outer tracks of a disk are longer than the inner tracks, the data density drops off significantly for the outer tracks. Consequently, storage space is wasted on all but the innermost track when using the fixed frequency, fixed angular velocity method of data storage.
The constant data density method provides increased data storage capacity relative to the fixed frequency, fixed angular velocity method. Constant data density is achieved by either varying the write frequency as a function of radius while keeping the angular velocity constant or by varying the angular velocity of the disk as a function of radius while keeping the write frequency constant. Typically, the maximum data density is determined for the inside track and on all the remaining tracks the data is recorded at the same data density.
A drawback of either constant data density method is that read errors increase on the outermost tracks. Variations in magnetic characteristics of the magnetic systems affect the optimal data density from track to track across the disk. For example, variations in the flying height of the slider, the thickness of the magnetic medium, the linear speed of the slider in relation to the disk and the response of the read/write circuitry affect the optimal data density.
Both constant data density methods fail to consider these variations in magnetic characteristics. The consequence is that data is written in particular tracks at frequencies other than the frequency which would produce an optimal data density for that track. Typically, the optimal data density is lower for the outer tracks than for the inner tracks. Since the data density for the inner track is used as the constant for the data density across the disk increased read errors result in the outer tracks where the data density should be lower to account for the changed magnetic characteristics of the system.
To assure that readings from certain tracks do not result in unacceptable read error rates there is a need for a data storage method and apparatus which controls the frequency to produce optimized data density for each track based upon the varying magnetic characteristics on a disk